This invention relates to apparatus for drilling generally horizontal boreholes in a subterranean earth formation, and more particularly to a device for controlling the trajectory of such a borehole to maintain same within the upper and lower boundaries of a mineral bed such as a seam of coal.
The drilling of long generally horizontal gas drainage holes in coal beds is known in the art as a method for degasifying a coal bed in advance of mining. The gas drainage holes drilled in this manner are either vented or connected to a vacuum source to remove methane from a coal bed. The greatest problem encountered in drilling these gas relief holes is that of maintaining the bit trajectory within the coal seam such that the resulting holes are actually through the coal seam rather than through an overlying or underlying formation. Control of borehole trajectory is also important in exploratory drilling for coal or other minerals.